Black Holes

A black hole is a region in space where gravity is so strong that nothing, not even light, can escape. While they cannot be seen directly black holes can be detected by the effect their immense gravity has on nearby objects.

Artist's concept of a black hole.Credit: HEASARC/NASA

Black holes have such strong gravity because their mass (which is tens to millions times the mass of the Sun) is confined to an extremely small region. This is called the singularity, a region of infinite density and infinite smallness, that cannot be described by the normal laws of physics.

The boundary of a black hole is called the event horizon. It is an invisible boundary but once across it you are pulled in towards the black hole with no means of escape. Furthermore, nothing within the event horizon can be seen by the outside world. The radius of the event horizon (in kilometers) is equal to 3 times the mass of the black hole (in solar masses).

Inside the event horizon, gravity increases rapidly as you approach the singularity. The change in gravity is so strong, that say you were to cross the event horizon feet first, your feet, being closer to the singularity will be under more gravity than your head. You would be stretched length-ways and squeezed sideways to form a long thin string of spaghetti before being ripped completely apart!

And if that's not bizarre enough, black holes also change the pace of time. Our common sense drives us to believe that time is constant but time slows down when gravity is strong (as predicted by Einstein's Theory of General Relativity). Experiments comparing clocks at the surface of the Earth to clocks placed at the top of multi-storey buildings, in high-flying aircraft, and in space flights, have shown that the ground-level clocks that are closer to the Earth's centre of gravity, run a little bit slower.

If you were to watch someone cross a black hole's event horizon and they happened to be carrying a clock you (the observer) would see the clock's second hand slow to an almost stop. The ill-fated person would appear frozen in time, hovering near the event horizon.

But the person falling would have no such reprieve. And in fact, if they tried to slow down it would only make things worse. For time also slows when accelerating. So the best advice for a person falling into a black hole is not to try and stop! By braking (decelerating) time speeds up and you approach the singularity faster than if you did nothing. Isn't physics strange!

Black holes do not roam the Universe devouring everything in sight. If the Sun was to suddenly be replaced by a black hole with the same mass as the Sun the Earth would continue to orbit as if nothing had happened. We would, however, notice the distinct lack of heat and sunlight!

i watched the planetarium show on Black Holes, and i was wondering is there a way i can get the transcript of the show? i am doing an assignment on black holes for school and i found the information from the show very helpful but i have also forgotten a lot. thanks

wow, kinda scary, but i heard from someone that if a black hole was to suk up a planet it would grow bigger and bigger the more it pulled in, is it true? or does a black hole stay the same size its whole life?

Hi there. Black holes can certainly get bigger and presumably they can also fade away. We measure the size of a black hole by its event horizon as described above. This is the point of no return, where gravity becomes too strong for anything, even light, to escape. The size of the event horizon is directly linked to the mass of the black hole. A black hole with the same mass as the Sun has an event horizon with a radius of 3km – and this scales, so that a black hole with 10 times the mass of the Sun will have a 30km radius event horizon, and so on.

So as a black hole pulls material into it, its mass increases and therefore so does its event horizon. And of course, the more it pulls in, the greater its mass, and then the more gravity it has to pull more material into it.

But don’t get too worried – the amazing things about black holes is that all the mass is confined to such a small area. If our Sun was to suddenly turn into a black hole (not really possible, but useful to think about), then instead of its mass being spread out over a diameter of 1.7million km it would shrink to just 6km in diameter. Much harder for something to fall into a 6km hole, than a 1.7million km one!

It is also thought that black holes can disappear – by giving off Hawking radiation. It’s a process proposed by Stephen Hawking using quantum mechanics. It’s kind of strange but in a vacuum quantum mechanics tells us that virtual particles are always appearing, then disappearing. If a virtual particle pair were to form near the event horizon of a black hole, one particle could be captured by the black hole, while the other escapes. The escaped particle needs energy and gains that from the black hole – so the ultimate cost to the black hole is a loss of energy (or mass). It’s a slow process but eventually a black hole could evaporate in this way. For more info see the NASA website.

Hi Chris, Yes, current understanding has one super massive black hole dubbed Sagittarius A* at the centre of our galaxy (and likely one in the centre of all major galaxies). It's inferred from long-term study of the motion of stars that appear to orbit an object at galactic centre that has a massive gravitational effect on them but which cannot be seen directly, and also from X-ray flares given off by surrounding stars and gas as they are drawn in and heated to extreme temperatures. The object would have the mass equivalent of at least 2 million Suns!